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Answer to Activity #1
Trihybrid Cross (three genes)
ABD
ABd
AbD
Abd
aBD
aBd
abD
abd
ABD
AABBDD
AABBDd
AABbDD
AABbDd
AaBBDD
AaBBDd
AaBbDD
AaBbDd
ABd
AABBDd
AABBdd
AABbDd
AABbdd
AaBBDd
AaBBdd
AaBbDd
AaBbdd
AbD
AABbDD
AABbDd
AAbbDD
AAbbDd
AaBbDD
AaBbDd
AabbDD
AabbDd
Abd
AABbDd
AABbdd
AAbbDd
AAbbdd
AaBbDd
AaBbdd
AabbDd
Aabbdd
aBD
AaBBDD
AaBBDd
AaBbDD
AaBbDd
aaBBDD
aaBBDd
aaBbDD
aaBbDd
aBd
AaBBDd
AaBBdd
AaBbDd
AaBbdd
aaBBDd
aaBBdd
aaBbDd
aaBbdd
abD
AaBbDD
AaBbDd
AabbDD
AabbDd
aaBbDD
aaBbDd
aabbDD
aabbDd
abd
AaBbDd
AaBbdd
AabbDd
Aabbdd
aaBbDd
aaBbdd
aabbDd
aabbdd
3
3
1
Phenotypic Ratio
27
9
9
9
3
Non- Mendelian
INHERITANCE
Many other patterns of inheritance not based on
Mendel's laws alone were discovered in plant
and animals.
Gene interactions are of two types intragenic and
intergenic.
Intergenic
Intragenic
, also
interactions, also interactions
known as non-allelic
known as inter-allelic
interactions occur
between the two alleles of
the same gene.
interactions occur between
alleles of different genes
located on the same or
different chromosomes.
Examples are incomplete
dominance and codominance.
examples are complementary
genes, supplementary genes,
epistasis, pleiotropy and lethal
genes.
Incomplete Dominance
•
interaction where both the alleles of a given trait is
expressed as a blend (mixture) as against to a normal
Mendelian pattern.
•
• an intermediate character is expressed. (This situation
occurs due to the fact that the dominant gene can not
completely suppress the expression of recessive gene. )
• With the result, the heterozygous offspring will be
phenotypically and genotypically different from either of the
homozygous parent
•
fig. 1. Flower Colour in Four O Clock Plant This example very clearly
indicates the phenomenon of incomplete dominance That the genes
responsible for red and white flowers do not actually mix, since both the
pure characters reappear in the F2 generation That there is no specific
gene responsible for producing pink flowers
Multiple Alleles
•
a condition where more than two genes occupy the same locus,
on the same pair of homologous chromosomes, in different
organisms. Each of these genes expresses a totally different
character. The inheritance of A B O blood groups in man is an
example of multiple alleles.
The fruit-fly, Drosphila melanogaster has 15 alleles for eye colour. In rabbits, there are 4 alleles for colour. In all these
cases, at any given time, only two of the alleles can occupy the same locus on a pair of homologous chromosomes.
Polygenic traits
(Polygenes)
• They are the result of the interaction of several genes.
• For instance, phenotypes like high blood pressure
(hypertension) are not the result of a single "blood
pressure" gene with many alleles (a 120/80allele, a 100/70
allele, a 170/95 allele, etc.)
• The phenotype is an interaction between a person's weight
(one or more obesity genes),cholesterol level (one or more
genes controlling metabolism), kidney function (salt
transporter genes), smoking (a tendency to addiction), and
probably lots of factors too.
• Each of the contributing genes can also have multiple
alleles.
Codominance
•
Co-dominance represents a situation where two allelic genes
when present together in an individual, express their traits
independently instead of showing a typical dominant recessive
relationship.
•
A classical example of codominance is the expression of blood
group AB. The genotype for this blood group is IA Ib. Each of the
two genes produces the respective antigen and neither of them
checks the expression of the other.
Heterodominance
(inbreeding, outbreeding and hybrid vigour)
• When the heterozygotes have a more extreme phenotype
than either of the corresponding homozygotes
(homozygous parents), then it is usually referred to as
overdominance, superdominance or heterodominance
(Serra, 1959).
• For example, there is heterodominance, when the
heterozygote Aa between a pair of factors which control
size is bigger than the homozygotes AA or aa. This type of
allelic relation which implies interaction between the
alleles, or of these with other factors of the genotype, may
be found in qualitative characters and especially those
such as size, production, vigour, etc., which are of
importance in the breeding of animals and plants
Epistasis
(Epi = above/over gene,
static = standing)
•
It is a pattern of inheritance
where a pair of genes situated
at one locus, prevent the
expression of a pair of genes
situated at another locus.
•
Such genes are called
inhibiting genes or epistatic
genes (Epi = above/over
gene, static = standing).
•
Epistasis reduces the number
of phenotypes in the F2
generation of a dihybrid
cross. A classical example of
epistasis is seen in the white
fowls.
WHITE FOWL: COLOURED FOWL 13 : 3 gene C
produces colour in the feathers. But the inhibiting gene
I prevents the appearance of colour. As a result the leg
horn fowl is white CCII. The plymouth rock fowl is
white since it has recessive genes ccii
Pleiotropy
•
It is a phenomenon where a given gene has multiple phenotypic
effects. As a result, the gene not only influences the trait which it
expresses, but also influences many other traits.
•
Pleiotropic genes generally do not have the same influence on all
the traits that they control. A pleiotropic gene may cause an
evident expression of its specific trait, representing a major effect
or a less evident expression of its other traits, representing a
secondary effect.
•
in the sweet pea plant, the genes that control the colour of the flower also
control the colour of the seed coat and appearance of red spots in the
axils of the leaves. In Drosophila, the gene that controls wing size also
affects the nature of balancers, eye colour, fertility and life span of the
insect.
Lethality and Lethal Alleles
•
The term Iethal is applied to those
changes in the genome of an organism
which produce effects severe enough to
cause death.
•
Lethality is a condition in which death of
a certain genotype occurs prematurely.
The fully dominant lethal allele kills the
carrier individual both in its homozygous
and heterozygous conditions.
•
The recessive lethal allele kills the carrier
individual only in homozygous condition.
They maybe of two kinds (i) one which has
no obvious phenotypic effect in
heterozygotes and (ii) one which exhibits a
distinctive phenotype when in heterozygous
condition.
Heritability:
• Heritability has two definitions. The first is a
statistical definition, and it defines heritability as
the proportion of phenotypic variance
attributable to genetic variance.
• The second definition is more common
"sensical". It defines heritability as the extent to
which genetic individual differences contribute to
individual differences in observed behavior (or
phenotypic individual differences)
• A heritable trait is most simply an offspring's trait
that resembles the parents' corresponding trait.
Penetrance
• is a term used in genetics describing the proportion of
individuals carrying a particular variation of a gene (an
allele or genotype) that also express an associated trait
(the phenotype).
• In medical genetics, the penetrance of a disease causing
mutation is the proportion of individuals with the mutation
who exhibit clinical symptoms.
• For example, if a mutation in the gene responsible for a
particular autosomal dominant disorder has 95%
penetrance, then 95% of those with the mutation develop
the disease, while 5% do not.
Examples of Incomplete
Penetrance
I.
Polydactyly in man is thought to be produced
by a dominant gene P. The normal condition with
five digits on each limb is produced by the
recessive genotype (pp). Some heterozygous
individuals (Pp) are not polydactylus and
therefore has a penetrance of less than 70%.
•
2. In man, the tendency to develop diabetes
mellitus (a condition in which there is an excess
of sugar in the blood) is controlled by certain
genes.
• However, not everyone carrying the gents for
diabetes actually develops the condition, for the
genes have incomplete penetrance.
• Effects of environment on penetrance- The
environmental factors and genetical background have
some definite effect on the degree of penetrance of a
gene. Accordingly, the percentage of penetrance of a
given gene may be altered by changing the conditions of
temperature, moisture, nutrition and so forth, under which
the organism develops.
• For example, when various twins which carry genes for
diabetes mellitus are studied, it is found that the disease
appears only in those cases with one eating more
carbohydrate foods (starch and sugars).
Variable expressivity
• Although some genetic disorders exhibit little variation,
most have signs and symptoms that differ among affected
individuals.
•
Variable expressivity refers to the range of signs and
symptoms that can occur in different people with the same
genetic condition.
•
For example, the features of Marfan syndrome vary widely—
some people have only mild symptoms (such as being tall and
thin with long, slender fingers), while others also experience lifethreatening complications involving the heart and blood vessels.
•
As with reduced penetrance, variable expressivity is probably caused by
a combination of genetic, environmental, and lifestyle factors, most of
which have not been identified. If a genetic condition has highly variable
signs and symptoms, it may be challenging to diagnose.
LATE ONSET
• Some autosomal dominant diseases do not express
themselves until later in life, well beyond the reproductive
years. The individuals who will develop the disease have
passed the mutant allele along to their offspring before
they themselves know they are affected.
•
In some cases even grandchildren are born before the
affected grandparent shows the first signs of the disease.
•
Huntington disease, sometimes called Huntington's Chorea because of
the choreic movements expressed as the disease progresses, is a good
example of a late onset disease. Age of onset varies from the teens to
the late sixties, with a mean age of onset between ages 35 and 45.
Nearly 100% of the individuals born with the defective allele will develop
the disease by the time they are 70. The disease is progressive with
death usually occurring between four and twenty-five years after the first
symptoms develop. Emotional changes often are the first symptoms.